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How Our Brains Replay Memories

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Retrieving a Memory
Caption: Encoding and replaying learned memory. Left panel shows the timed sequence of neurons firing in a part of a person’s brain involved in memory as it encodes the random pair of words, “crow” and “jeep.” Colors are assigned to different neurons to differentiate their firing within the sequence. Right panel shows a highly similar timed sequence of those same neurons firing just before a person given the word “jeep,” recalled and said the correct answer “crow.” Credit: Vaz AP, Science, 2020.

Note to my blog readers: the whole world is now facing a major threat from the COVID-19 pandemic. We at NIH are doing everything we can to apply the best and most powerful science to the development of diagnostics, therapeutics, and vaccines, while also implementing public health measures to protect our staff and the patients in our hospital. This crisis is expected to span many weeks, and I will occasionally report on COVID-19 in this blog format. Meanwhile, science continues to progress on many other fronts—and so I will continue to try to bring you stories across a wide range of topics. Perhaps everyone can use a little break now and then from the coronavirus news? Today’s blog takes you into the intricacies of memory.

When recalling the name of an acquaintance, you might replay an earlier introduction, trying to remember the correct combination of first and last names. (Was it Scott James? Or James Scott?) Now, neuroscientists have found that in the split second before you come up with the right answer, your brain’s neurons fire in the same order as when you first learned the information [1].

This new insight into memory retrieval comes from recording the electrical activity of thousands of neurons in the brains of six people during memory tests of random word pairs, such as “jeep” and “crow.” While similar firing patterns had been described before in mice, the new study is the first to confirm that the human brain stores memories in specific sequences of neural activity that can be replayed again and again.

The new study, published in the journal Science, is the latest insight from neurosurgeon and researcher Kareem Zaghloul at NIH’s National Institute of Neurological Disorders and Stroke (NINDS). Zaghloul’s team has for years been involved in an NIH Clinical Center study for patients with drug-resistant epilepsy whose seizures cannot be controlled with drugs.

As part of this work, his surgical team often temporarily places a 4 millimeter-by-4 millimeter array of tiny electrodes on the surface of the brains of the study’s participants. They do this in an effort to pinpoint brain tissues that may be the source of their seizures before performing surgery to remove them. With a patient’s informed consent to take part in additional research, the procedure also has led to a series of insights into what happens in the human brain when we make and later retrieve new memories.

Here’s how it works: The researchers record electrical currents as participants are asked to learn random word pairs presented to them on a computer screen, such as “cake” and “fox,” or “lime” and “camel.” After a period of rest, their brain activity is again recorded as they are given a word and asked to recall the matching word.

Last year, the researchers reported that the split second before a person got the right answer, tiny ripples of electrical activity appeared in two specific areas of the brain [2]. The team also had shown that, when a person correctly recalled a word pair, the brain showed patterns of activity that corresponded to those formed when he or she first learned to make a word association.

The new work takes this a step further. As study participants learned a word pair, the researchers noticed not only the initial rippling wave of electricity, but also that particular neurons in the brain’s cerebral cortex fired repeatedly in a sequential order. In fact, with each new word pair, the researchers observed unique firing patterns among the active neurons.

If the order of neuronal firing was essential for storing new memories, the researchers reasoned that the same would be true for correctly retrieving the information. And, indeed, that’s what they were able to show. For example, when individuals were shown “cake” for a second time, they replayed a very similar firing pattern to the one recorded initially for this word just milliseconds before correctly recalling the paired word “fox.”

The researchers then calculated the average sequence similarity between the firing patterns of learning and retrieval. They found that as a person recalled a word, those patterns gradually became more similar. Just before a correct answer was given, the recorded neurons locked onto the right firing sequence. That didn’t happen when a person gave an incorrect answer.

Further analysis confirmed that the exact order of neural firing was specific to each word pair. The findings show that our memories are encoded as unique sequences that must be replayed for accurate retrieval, though we still don’t understand the molecular mechanisms that undergird this.

Zaghloul reports that there’s still more to learn about how these processes are influenced by other factors such as our attention. It’s not yet known whether the brain replays sequences similarly when retrieving longer-term memories. Along with these intriguing insights into normal learning and memory, the researchers think this line of research will yield important clues as to what changes in people who suffer from memory disorders, with potentially important implications for developing the next generation of treatments.

Reference:

[1] Replay of cortical spiking sequences during human memory retrieval. Vaz AP, Wittig JH Jr, Inati SK, Zaghloul KA. Science. 2020 Mar 6;367(6482):1131-1134.

[2] Coupled ripple oscillations between the medial temporal lobe and neocortex retrieve human memory. Vaz AP, Inati SK, Brunel N, Zaghloul KA. Science. 2019 Mar 1;363(6430):975-978.

Links:

Epilepsy Information Page (National Institute of Neurological Disorders and Stroke/NIH)

Brain Basics (NINDS)

Zaghloul Lab (NINDS)

NIH Support: National Institute of Neurological Disorders and Stroke; National Institute of General Medical Sciences


Discovering the Brain’s Nightly “Rinse Cycle”

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Getting plenty of deep, restful sleep is essential for our physical and mental health. Now comes word of yet another way that sleep is good for us: it triggers rhythmic waves of blood and cerebrospinal fluid (CSF) that appear to function much like a washing machine’s rinse cycle, which may help to clear the brain of toxic waste on a regular basis.

The video above uses functional magnetic resonance imaging (fMRI) to take you inside a person’s brain to see this newly discovered rinse cycle in action. First, you see a wave of blood flow (red, yellow) that’s closely tied to an underlying slow-wave of electrical activity (not visible). As the blood recedes, CSF (blue) increases and then drops back again. Then, the cycle—lasting about 20 seconds—starts over again.

The findings, published recently in the journal Science, are the first to suggest that the brain’s well-known ebb and flow of blood and electrical activity during sleep may also trigger cleansing waves of blood and CSF. While the experiments were conducted in healthy adults, further study of this phenomenon may help explain why poor sleep or loss of sleep has previously been associated with the spread of toxic proteins and worsening memory loss in people with Alzheimer’s disease.

In the new study, Laura Lewis, Boston University, MA, and her colleagues at the Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston. recorded the electrical activity and took fMRI images of the brains of 13 young, healthy adults as they slept. The NIH-funded team also built a computer model to learn more about the fluid dynamics of what goes on in the brain during sleep. And, as it turns out, their sophisticated model predicted exactly what they observed in the brains of living humans: slow waves of electrical activity followed by alternating waves of blood and CSF.

Lewis says her team is now working to come up with even better ways to capture CSF flow in the brain during sleep. Currently, people who volunteer for such experiments have to be able to fall asleep while wearing an electroencephalogram (EEG) cap inside of a noisy MRI machine—no easy feat. The researchers are also recruiting older adults to begin exploring how age-related changes in brain activity during sleep may affect the associated fluid dynamics.

Reference:

[1] Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Fultz NE, Bonmassar G, Setsompop K, Stickgold RA, Rosen BR, Polimeni JR, Lewis LD. Science. 2019 Nov 1;366(6465):628-631.

Links:

Sleep and Memory (National Institute of Mental Health/NIH)

Sleep Deprivation and Deficiency (National Heart, Lung, and Blood Institute/NIH)

Alzheimer’s Disease and Related Dementias (National Institute on Aging/NIH)

NIH Support: National Institute of Mental Health; National Institute of Biomedical Imaging and Bioengineering; National Institute of Neurological Disorders and Stroke


A Real-Time Look at Value-Based Decision Making

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All of us make many decisions every day. For most things, such as which jacket to wear or where to grab a cup of coffee, there’s usually no right answer, so we often decide using values rooted in our past experiences. Now, neuroscientists have identified the part of the mammalian brain that stores information essential to such value-based decision making.

Researchers zeroed in on this particular brain region, known as the retrosplenial cortex (RSC), by analyzing movies—including the clip shown about 32 seconds into this video—that captured in real time what goes on in the brains of mice as they make decisions. Each white circle is a neuron, and the flickers of light reflect their activity: the brighter the light, the more active the neuron at that point in time.

All told, the NIH-funded team, led by Ryoma Hattori and Takaki Komiyama, University of California at San Diego, La Jolla, made recordings of more than 45,000 neurons across six regions of the mouse brain [1]. Neural activity isn’t usually visible. But, in this case, researchers used mice that had been genetically engineered so that their neurons, when activated, expressed a protein that glowed.

Their system was also set up to encourage the mice to make value-based decisions, including choosing between two drinking tubes, each with a different probability of delivering water. During this decision-making process, the RSC proved to be the region of the brain where neurons persistently lit up, reflecting how the mouse evaluated one option over the other.

The new discovery, described in the journal Cell, comes as something of a surprise to neuroscientists because the RSC hadn’t previously been implicated in value-based decisions. To gather additional evidence, the researchers turned to optogenetics, a technique that enabled them to use light to inactivate neurons in the RSC’s of living animals. These studies confirmed that, with the RSC turned off, the mice couldn’t retrieve value information based on past experience.

The researchers note that the RSC is heavily interconnected with other key brain regions, including those involved in learning, memory, and controlling movement. This indicates that the RSC may be well situated to serve as a hub for storing value information, allowing it to be accessed and acted upon when it is needed.

The findings are yet another amazing example of how advances coming out of the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative are revolutionizing our understanding of the brain. In the future, the team hopes to learn more about how the RSC stores this information and sends it to other parts of the brain. They note that it will also be important to explore how activity in this brain area may be altered in schizophrenia, dementia, substance abuse, and other conditions that may affect decision-making abilities. It will also be interesting to see how this develops during childhood and adolescence.

Reference:

[1] Area-Specificity and Plasticity of History-Dependent Value Coding During Learning. Hattori R, Danskin B, Babic Z, Mlynaryk N, Komiyama T. Cell. 2019 Jun 13;177(7):1858-1872.e15.

Links:

Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)

Komiyama Lab (UCSD, La Jolla)

NIH Support: National Institute of Neurological Disorders and Stroke; National Eye Institute; National Institute on Deafness and Other Communication Disorders


What a Memory Looks Like

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Engram Image
Credit: Stephanie Grella, Ramirez Group, Boston University

Your brain has the capacity to store a lifetime of memories, covering everything from the name of your first pet to your latest computer password. But what does a memory actually look like? Thanks to some very cool neuroscience, you are looking at one.

The physical manifestation of a memory, or engram, consists of clusters of brain cells active when a specific memory was formed. Your brain’s hippocampus plays an important role in storing and retrieving these memories. In this cross-section of a mouse hippocampus, imaged by the lab of NIH-supported neuroscientist Steve Ramirez, at Boston University, cells belonging to an engram are green, while blue indicates those not involved in forming the memory.

When a memory is recalled, the cells within an engram reactivate and turn on, to varying degrees, other neural circuits (e.g., sight, sound, smell, emotions) that were active when that memory was recorded. It’s not clear how these brain-wide connections are made. But it appears that engrams are the gatekeepers that mediate memory.

The story of this research dates back several years, when Ramirez helped develop a system that made it possible to image engrams by tagging cells in the mouse brain with fluorescent dyes. Using an innovative technology developed by other researchers, called optogenetics, Ramirez’s team then discovered it could shine light onto a collection of hippocampal neurons storing a specific memory and reactivate the sensation associated with the memory [1].

Ramirez has since gone on to show that, at least in mice, optogenetics can be used to trick the brain into creating a false memory [2]. From this work, he has also come to the interesting and somewhat troubling conclusion that the most accurate memories appear to be the ones that are never recalled. The reason: the mammalian brain edits—and slightly changes—memories whenever they are accessed.

All of the above suggested to Ramirez that, given its tremendous plasticity, the brain may possess the power to downplay a traumatic memory or to boost a pleasant recollection. Toward that end, Ramirez’s team is now using its mouse system to explore ways of suppressing one engram while enhancing another [3].

For Ramirez, though, the ultimate goal is to develop brain-wide maps that chart all of the neural networks involved in recording, storing, and retrieving memories. He recently was awarded an NIH Director’s Transformative Research Award to begin the process. Such maps will be invaluable in determining how stress affects memory, as well as what goes wrong in dementia and other devastating memory disorders.

References:

[1] Optogenetic stimulation of a hippocampal engram activates fear memory recall. Liu X, Ramirez S, Pang PT, Puryear CB, Govindarajan A, Deisseroth K, Tonegawa S. Nature. 2012 Mar 22;484(7394):381-385.

[2] Creating a false memory in the hippocampus. Ramirez S, Liu X, Lin PA, Suh J, Pignatelli M, Redondo RL, Ryan TJ, Tonegawa S. Science. 2013 Jul 26;341(6144):387-391.

[3] Artificially Enhancing and Suppressing Hippocampus-Mediated Memories. Chen BK, Murawski NJ, Cincotta C, McKissick O, Finkelstein A, Hamidi AB, Merfeld E, Doucette E, Grella SL, Shpokayte M, Zaki Y, Fortin A, Ramirez S. Curr Biol. 2019 Jun 3;29(11):1885-1894.

Links:

The Ramirez Group (Boston University, MA)

Ramirez Project Information (Common Fund/NIH)

NIH Director’s Early Independence Award (Common Fund)

NIH Director’s Transformative Research Award (Common Fund)

NIH Support: Common Fund


New Grants Explore Benefits of Music on Health

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It’s not every day you get to perform with one of the finest voices on the planet. What an honor it was to join renowned opera singer Renée Fleming back in May for a rendition of “How Can I Keep from Singing?” at the NIH’s J. Edward Rall Cultural Lecture. Yet our duet was so much more. Between the song’s timeless message and Renée’s matchless soprano, the music filled me with a profound sense of joy, like being briefly lifted outside myself into a place of beauty and well-being. How does that happen?

Indeed, the benefits of music for human health and well-being have long been recognized. But biomedical science still has a quite limited understanding of music’s mechanisms of action in the brain, as well as its potential to ease symptoms of an array of disorders including Parkinson’s disease, stroke, and post-traumatic stress disorder (PTSD). In a major step toward using rigorous science to realize music’s potential for improving human health, NIH has just awarded $20 million over five years to support the first research projects of the Sound Health initiative. Launched a couple of years ago, Sound Health is a partnership between NIH and the John F. Kennedy Center for the Performing Arts, in association with the National Endowment for the Arts.

With support from 10 NIH institutes and centers, the Sound Health awardees will, among other things, study how music might improve the motor skills of people with Parkinson’s disease. Previous research has shown that the beat of a metronome can steady the gait of someone with Parkinson’s disease, but more research is needed to determine exactly why that happens.

Other fascinating areas to be explored by the Sound Health awardees include:

• Assessing how active music interventions, often called music therapies, affect multiple biomarkers that correlate with improvement in health status. The aim is to provide a more holistic understanding of how such interventions serve to ease cancer-related stress and possibly even improve immune function.

• Investigating the effects of music on the developing brain of infants as they learn to talk. Such work may be especially helpful for youngsters at high risk for speech and language disorders.

• Studying synchronization of musical rhythm as part of social development. This research will look at how this process is disrupted in children with autism spectrum disorder, possibly suggesting ways of developing music-based interventions to improve communication.

• Examining the memory-related impacts of repeated exposures to a certain song or musical phrase, including those “earworms” that get “stuck” in our heads. This work might tell us more about how music sometimes serves as a cue for retrieving associated memories, even in people whose memory skills are impaired by Alzheimer’s disease or other cognitive disorders.

• Tracing the developmental timeline—from childhood to adulthood—of how music shapes the brain. This will include studying how musical training at different points on that timeline may influence attention span, executive function, social/emotional functioning, and language skills.

We are fortunate to live in an exceptional time of discovery in neuroscience, as well as an extraordinary era of creativity in music. These Sound Health grants represent just the beginning of what I hope will be a long and productive partnership that brings these creative fields together. I am convinced that the power of science holds tremendous promise for improving the effectiveness of music-based interventions, and expanding their reach to improve the health and well-being of people suffering from a wide variety of conditions.

Links:

The Soprano and the Scientist: A Conversation About Music and Medicine, (National Public Radio, June 2, 2017)

NIH Workshop on Music and Health, January 2017

Sound Health (NIH)

NIH Support: National Center for Complementary and Integrative Health; National Eye Institute; National Institute on Aging; National Institute on Alcohol Abuse and Alcoholism; National Institute on Deafness and Other Communication Disorders; National Institute of Mental Health; National Institute of Neurological Disorders and Stroke; National Institute of Nursing Research; Office of Behavioral and Social Sciences Research; Office of the Director


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